1. Field of Invention
This invention relates to the melting of scrap metal, particularly scrap aluminum, and more particularly to controlling pollutants emitted during the melting of scrap contaminated with oil, grease and other combustible hydrocarbon contaminants.
2. Description of the Prior Art
A common source of aluminum feed stock for remelting operations is scrap obtained from the industry. Such scrap includes turnings and borings from a machine shop, aluminum can scrap, oily aluminum briquettes, and miscellaneous skeletal scrap remaining after a sheet has been punched, blanked or stamped. Often this scrap is collected and compacted into scrap briquettes having a size of, for example, one foot cubed. This scrap is typically contaminated with oil, grease and other lubricants, or lacquers, paint and other metal coating materials decomposable or vaporizable at metal melting temperatures.
A major problem associated with the melting of such scrap involves the elimination or effective removal of hydrocarbon fumes emitted from the scrap in a remelt furnace. It is common practice to melt such scrap by introducing the scrap into a hot molten aluminum bath or stream. Scrap aluminum briquettes are less dense than molten aluminum which causes the briquettes to float on the surface on the molten aluminum. In such a situation, if the furnace temperature is too high, the aluminum scrap exposed to the heat and air will oxidize at an accelerated rate resulting in excessive melt loss, as discussed in Spear et al U.S. Pat. No. 3,770,420.
It has been disclosed in the prior art, such as Habayeb U.S. Pat. No. 3,869,112, to construct a remelt furnace with two sections, a charging well and a melting chamber constructed such that molten metal at the bottom of the furnace may circulate therebetween. The charging well is isolated from the burners which provide hot blast to the melting chamber and therefore the oxidation rate and resultant melt loss are reduced. However, the temperature in the charging well is not high enough to burn the hydrocarbon fumes emitted from the scrap in the charging well. An apparatus, including a collecting hood, a duct, and a blower are provided to convey the emissions from the charging well to the burners in the melting chamber where the hydrocarbons in the emissions are burned.
Heretofore, conventional open hearth and other remelt furnaces were constructed to operate under positive pressure in order to prevent air from entering the furnace. Air has been considered detrimental to the melting operation in two ways. First, the ambient air has a cooling effect on the furnace. Second, the air causes excessive oxidation of the scrap with resulting melt loss. Even with the advent of multi-chamber furnaces the practice of operating both chambers under positive pressure to avoid ingesting air persisted. Maintaining such positive pressure tends to require the use of auxiliary equipment to convey hydrocarbon vapors from a charging chamber to a heating chamber, as taught in Rawlings U.S. Pat. No. 3,839,016 and Habayeb U.S. Pat. No. 3,869,112. This type of complex equipment, including blowers, ductwork and associated instruments and controls, involves not only substantial initial capital expenditures but also significant maintenance and operating expenses which are desirably avoided or at least reduced.
Accordingly, an economical process and apparatus is desired for eliminating or effectively removing hydrocarbon fumes emitted from contaminated scrap in a remelt furnace and utilizing the fuel values therein while avoiding economic and reliability shortcomings of the prior systems.